Like this article? We recommend

Introduction

For several years, DVD movie buffs have appreciated the benefits of
pre-recorded (pressed) dual-layer DVD media. Dual-layer DVD media enables movies
of up to 4 hours (240 minutes) in length to be played without disc-swapping, and
provides nearly twice the room on each disc for bonus features. However, until
the development of double layer rewritable DVD drives and media in 2004, PC
users could read, but could not create, dual-layer discs. In this article, you
learn how double layer DVD rewritable drives work, how to enable older
single-layer DVD drives to read double layer media, and which vendors produce
double layer DVD drives and DVD media.

The first (and so far, only) double layer rewritable standard comes from the
DVD+RW Alliance
(www.dvdrw.com): DVD+R
DL, although all drives capable of writing the DVD+R DL format will also be able
to write the future DVD-R DL format when it is eventually released. Sony was the
first to introduce a dual-layer drive (the DRU-700A) in March 2004, however many
manufacturers including BenQ, LiteON, Philips, Pioneer, NEC, and others have
since followed suit.

MediaTek is the world's largest supplier of optical drive chipsets, and
supplies the chipsets driving most of the DVD burners on the market, including
double layer models. Other companies including NEC, Philips, Sanyo and TDK also
provide chipsets for double layer drives.

It is interesting to note that the DL in DVD+R DL actually stands for
Double Layer and not Dual-Layer, however the terms are often
interchanged, and some media is advertised using both terms. The DVD+RW Alliance
uses the Double Layer terminology to help prevent confusion with the fact
that most DVD burners are also capable of reading and writing Dual Format (+ and
-) media. Since the term Dual-Layer was originally used with pressed
DVD-9 discs, it seems that many are maintaining the use of that terminology for
pressed DVD-9 discs, while reserving the Double Layer designation for
writable or rewritable discs. In the end, the only important thing to note is
that for all intents and purposes the terms are identical, and are often used
interchangeably.

How Dual-Layer DVD Media Works

Dual-layer DVD-Video media has been used for several years by the DVD movie
industry. Dual-layer DVDs have a thin substrate layer between the first and
second layers of DVD data. Single-sided dual-layer DVD-Video media is known
as DVD-9. Note that DVD+R DL is also referred to as DVD+R9. It has a capacity
of 8.5GB (slightly less than twice the 4.7GB capacity of standard DVD-5 media).
It's also possible to have double-sided dual-layer DVD-Video media, known
as DVD-18; some movie vendors use this technology to place widescreen (letterbox)
and fullscreen (4:3 TV format) versions of a film on a single DVD. The user
inserts the media into the DVD player or drive with the widescreen side up to
view the film in its original format. By inserting the media with the fullscreen
side up, the film can be viewed in its modified TV format. Almost all DVD set-top
boxes and DVD-ROM drives except for possibly some very old models can read DVD-9
and DVD-18 media. Inside the drive, a single laser is refocused when switching
between layers, accounting for a slight delay when moving from the top to the
bottom layer. See Chapter 13 my new book Upgrading
and Repairing PCs, 16th Edition, for complete technical
specifications and diagrams of DVD media.

How Double Layer DVD Rewritable Drives Work

DVD+R DL discs use a single refocusable laser to write both layers. The top
layer (Layer 0; L0) is written first. The metal reflector used by L0 is
semi-transparent, enabling the refocused laser to write to the second layer
(Layer 1; L1). Because the L0 layer absorbs some of the laser's energy,
only about half the laser power reaches the L1 layer.